Question

The average age of three brothers is 15 years. if the age of two brother is 15 and 12 years. find the age of third brother ​

Answer 1

Let the total age of brothers be 'y'

Let the age of 3rd brother be 'x'

ATQ

15 = y/3

y = 45 years

Total age = 45 years

15 + 12 + x = 45

27 + x = 45

x = 18 years

Age of the 3rd brother is 18 years.

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Answer 2

15 + 15 + 15 = 45

x + x + x = 45

You know 2 of the x’s. 15 and 12.

12 + 15 + x = 45

27 + x = 45

Move the 27 to isolate x. Use subraction.

x = 45 - 27

x = 18

Another way,

15 15 15 averages 15. We know this.

12 15 x also averages 15.

if 12 is 3 less than 15, and one of your answers IS 15, your other answer is 3 higher than 15.

12 15 18.

12 (15–3), 15, 18 (15+3)